1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly relates to a semiconductor device of the ball grid array (hereinafter referred to as BGA) type and a method of forming the same.
In recent years, there has been a significant increase in circuit density of semiconductor devices, and, also, there has been an increasing demand for a higher density in implementing semiconductor devices. This leads to more attention being paid on semiconductor devices of the BGA type, which can make intervals of nodes wider than can those of the QFP type, and, also, can be implemented more closely to each other.
There has been also an increase in frequencies of signals processed by the semiconductor devices. Thus, semiconductor devices of the BGA type desirably have configurations which are suitable for the processing of high frequency signals.
Also, the amount of heat generated by semiconductor chips has a tendency to increase as the semiconductor chips are made in higher circuit densities. Thus, semiconductor devices of the BGA type are required to have structures which are suitable for releasing heat.
2. Description of the Related Art
FIG. 1 shows a semiconductor device of the BGA type disclosed in the U.S. Pat. No. 5,166,772.
A semiconductor device 10 includes an assembly board 11, a semiconductor chip 12 fixed to a central portion of an upper surface of the assembly board 11, solder balls 13 arranged at nodes of a grid pattern on a lower surface of the assembly board 11, and a cover 14 made of a resin for sealing the semiconductor chip 12.
In FIG. 1, wires 17 couple between pads 15 on the semiconductor chip 12 and pads 16 on the assembly board 11.
FIG. 2 shows an enlarged plan view of the assembly board 11 of FIG. 1. FIG. 3 shows a cross-sectional view of the assembly board 11 taken along a line shown as A--A in FIG. 2. As shown in FIG. 3, the assembly board 11 includes a printed board 20 and an additive layer 21 provided on the printed board 20. The assembly board 11 also includes through-holes 22.sub.1 to 22.sub.5.
Each of the pads 15 on the semiconductor chip 12 is electrically connected to a corresponding one of the solder balls 13 provided beneath the assembly board 11. For example, as can be seen in FIG. l, FIG. 2, and FIG. 3, a pad 15.sub.1 is connected to a solder ball 13.sub.1 via a wire 17.sub.1, a pad 16.sub.1, a wiring pattern 23 on the additive layer 21, and a through-hole inner layer 24 provided on an inner surface of the through-hole 22.sub.2.
In this configuration, the through-holes 22.sub.1 to 22.sub.5 are open holes, so that the additive layer 21 cannot be formed at the locations of the through-holes 22.sub.1 to 22.sub.5. Thus, wiring patterns on the additive layer 21 cannot be formed in such manner that they traverse the locations of the through-holes 22.sub.1 to 22.sub.5.
This leads to a restriction on formation of wiring patterns such that paths of the wiring patterns cannot be laid freely.
Take an example of forming a wiring pattern connecting the pad 15.sub.1 and the through-hole 22.sub.2. A straight wiring pattern 25 as shown by dotted lines in FIG. 2 cannot be formed. Thus, the wiring pattern 23 ends up having a bending shape detouring from a straight line in order to avoid the through-holes 22.sub.1 and 22.sub.5.
As a result, the wiring pattern 23 is bound to have a longer path length than otherwise. This is undesirable in terms of signal propagation, since signals having high frequencies may be distorted in the longer path.
Since the semiconductor chip 12 is sealed by the cover 14, it is difficult for heat generated by the semiconductor chip 12 to be transferred to the outside. Namely, the semiconductor device 10 is not suitably structured in terms of releasing heat.
Accordingly, there is a need in the field of semiconductor devices for a semiconductor device in which there is no restriction on formation of wiring paths and heat can be released with ease, and for a method of forming that semiconductor device.
Also, there is a need for a semiconductor device in which there is a wider scope for an arrangement of wirings for external connection.